Imagine walking through Camden Town. But instead of eccentric markets and giggling tourists, there's an eight-lane motorway soaring on concrete pylons above a windswept wasteland. Or Dalston: no hip bars and warehouse conversions – just a gigantic spider's web road junction which has obliterated everything. Or Highbury: no delis or pubs; a great slab of grey trunk road instead. It's a similar story in Clapham, Kilburn, Deptford, Barnes, Canonbury, Balham, Battersea... the list goes on.

In the late 1960s, London was within a whisker of getting these motorways, and more. If the planners had had their way, they would have wrought more destruction than the Luftwaffe could manage when it bombed the capital every night for 76 nights in the autumn of 1940. The London Ringways project would have been the largest single construction project in British history.

Everyone knows about the Blitz. Yet hardly anyone is aware of the hidden history of these motorways – and this secret story of destruction and hope is almost as compelling as the stories of how the East End held out during Hitler's attempt to flatten the capital.

The problem: the post-war authorities feared that car ownership was getting out of control, and that the streets would soon be gridlocked. The plan: to ensnare the capital with circular motorways like a beast that needed taming – and then build more, penetrating from every point of Britain right into the centre of the city.

I became intrigued while living in Shepherd's Bush. I wondered why there was a mile-long motorway that didn't go anywhere. This road was known as the West Cross Route. At either end, Holland Park and White City roundabouts have slip roads, suggesting the motorway would have ploughed on north and south through dense housing. It turns out this was the plan; a small part of a monumental undertaking.

Roads enthusiast Chris Marshall is a Ringways expert: "The Motorway Box plan alone came with a price tag of billions of pounds – tens of billions in today's prices – and was put before the Cabinet as a single scheme. That made it, at the time – and probably, if adjusted for inflation, even now – the single biggest item of public expenditure ever proposed."

There had been ambitious plans to give London a new road network earlier in the century. Engineer Charles Bressey and architect Sir Edwin Lutyens knocked heads together in the 1930s, but the Second World War got in the way of their plans.

After that war, Britain's most famous town planner, Patrick Abercrombie, dreamt up another monumental vision involving wholescale redevelopment. It was the dream of every child with a Meccano kit and a Hornby train set. Abercrombie was a man obsessed by the future; and no one was going to stand in the way of progress. His 1940s plans contained provisions for various motorways – including one zooming past Buckingham Palace.

When the Euston Arch was demolished for a new train station in 1961, it seemed that nothing was sacred any more. Other world cities got their motorways: Paris had its Périphérique, while Robert Moses was carving freeways through the Bronx at will. The Greater London Council's (GLC) final assault on traffic jams was drawn up in secret and eventually published in 1966.

There were to be four concentric ring roads. The North Circular and the M25 were completed in the 1980s. But the innermost, Ringway 1 – dubbed the "Motorway Box", even though it looked more like a parcel the postman had squashed to fit through a letter box – was the real Trojan Horse: four interconnected motorways that would have caused 100,000 people to be evicted, and changed the lives of millions of Londoners. The North Cross Route was to slice from Harlesden to Hackney, the South Cross Route from Clapham Junction to Kidbrooke. The two parallel roads would be joined up by the West and East Cross Routes to form one bulbous, eight-lane ring road.

So what would a London cheese-wired up with elevated motorways have looked like? The longest completed section snakes down from Hackney to Blackheath. But there's also a surprising clue in Brixton. I meet Mike Slocombe – who runs the non-profit community blog Urban75.net – at Southwyck House, the so-called "Barrier Block". I'm surprised to find that Slocombe actually lives inside the building we've been chatting about on the phone. "You can understand why architects surrounded by Victorian buildings wanted to build something new," he tells me, as we survey the view from his balcony, "but it was motorway madness. People mistake the Barrier Block for Brixton Prison. It's not the prettiest, but it's distinctive." The South Cross Route was to have swept past on stilts, bashing its way through Brixton town centre, all of which was to be razed and replaced with 50 tower blocks.

The Barrier Block was constructed to deflect road noise, protecting houses behind it. The eerie empty area in front of the flats is where the motorway would have sat. Slocombe believes that "attitudes changed in the early 1970s. After the war, people just wanted a home, and progress – plus there was no money. They'd take what they could. Later, people began to ask questions, to challenge the authorities."

The Westway was also part of the plans. It's London's most famous urban motorway. When it was being pushed through North Kensington from 1964 to 1970, grass-roots protest movements emerged and politicians got cold feet about building copycat motorways. Chris Marshall notes: "What I find most impressive is the way in which the public fought the GLC. They were very astute in recognising that their two weapons were the ballot box and the public inquiry." In 1971, opposition movements coalesced into the London Motorway Action Group. It was the very start of the road protest movement – which mushroomed in the following 20 years.

There was also the small issue of cash. Marshall explains: "For the Treasury, it was just too much money. It came just after it saw off the fabulously expensive plans for the new Maplin airport in the Thames Estuary." In 1973, the Ringways project was finally put to the sword. "It was stirring up far too much bad publicity for the government to risk being seen to endorse it."

In the end, London had got off lightly. Almost every British town of the era was desperate for a hulking ring road. Coventry was gifted one so perfectly circular that the city looks like a dartboard on maps. Glasgow built half a motorway box, the M8. Even small towns such as Stourbridge had to have theirs. York escaped – there was uproar about a vast dual carriageway cheek by jowl with the city walls. Leeds built urban motorways like they were going out of fashion. It seems impossible to believe now, but the city council proudly dubbed Leeds the "Motorway City of the Seventies".

But it was Britain's Motown – the even more car-crazy Birmingham – that got the most new urban highways of all. The Milanese sculptor's son Herbert Manzoni ran Birmingham's planning department like a personal fiefdom. A real slash-and-burn pioneer who followed the city's motto – "Forward" – to the letter, Manzoni authorised the most complete inner-ring-road project in a British city. That road – the Queensway – was opened by the Queen in 1971, the year before nearby Spaghetti Junction on the M6. But the maintenance costs, noise, pollution and space these roads took up soon turned many against them. The Queensway is now somewhat ungallantly dubbed a "concrete collar" by Brum's turncoat council, who started successfully demolishing parts of it in the 1990s – lifting the city out of a motor malaise.

Back in London, it takes me an hour to get from Brixton to Hackney by public transport. It would have taken 10 minutes by Ringway. So maybe they did have an upside after all? From Hackney Wick station I cross a wobbly footbridge towards Victoria Park, and look down on the river of cars. With motorways, it's the sound that hits you – that never-ending low-frequency growl. This is the old East Cross Route (now the A12) – the most complete portion of the Motorway Box built. Looking south towards Blackwall Tunnel, I imagine all those vibrant London districts replaced with roads like this. Over the interminable din of traffic, I listen to Mike Slocombe's words from earlier in the day. "Thank fuck they didn't build the road through Brixton," he laughs. I swear I can hear a car horn as the expletive rolls around my head.